Effective use of electric power is required for recent global warming tendency. Secondary batteries for electric power storage are expected as one means, and from the standpoint of prevention of air pollution, early practical application of large secondary batteries is expected as an automobile power source. Further, a demand of small secondary batteries has been steadily increasing especially in association with spread and performance enhancement of electrical devices, such as digital cameras and mobile phones, as back-up sources of computers and power sources of small household electrical appliances.
As these secondary batteries, a secondary battery having performance corresponding to a device to be used is required, and typically, lithium ion batteries are mainly used.
In the lithium ion battery, a negative material formed such that a negative active material of graphite, or the like is fixed to a negative substrate made of a copper foil in a metal packaging can of aluminum, iron, or the like, a positive electrode material formed such that a positive electrode active material of lithium nickel oxide, lithium cobalt oxide, or the like is fixed to a positive electrode substrate made of an aluminum foil, a current collector made of aluminum or copper, a separator made of a resin film, such as a polypropylene porous film, an electrolyte solution, an electrolyte, and the like are enclosed.
By the way, establishment of measures against environment pollution with used lithium ion batteries is strongly required for an expanding demand of lithium ion batteries, and recovery and effective use of valuable metals have been examined.
As a method for recovering the valuable metals from a lithium ion battery having the above structure, for example, dry melting treatment and incineration treatment for discharging the battery and decomposing and removing a solvent, as described in Patent Literature 1, are often used. Patent Literature 1 discloses pretreatment to roast a lithium ion battery at a temperature of 350° C. or more, to perform pulverization, and then to perform screening.
However, in the case of dry treatment like the technology described in Patent Literature 1, consumption of energy and exhaust gas treatment are problems. Further, especially, in a melting method, lithium is made into slag and becomes unrecoverable, and in a roasting method, contained phosphorus and fluorine are fixed as a water-insoluble phosphate and fluoride. Lithium, cobalt, nickel, or the like that are valuable metals are contaminated. As a result, separation and refinement become difficult. Therefore, direct regeneration of lithium, cobalt, and nickel recovered in dry roasting treatment for battery materials is difficult in terms of quality, and effective reuse cannot be achieved.
Meanwhile, methods of recovering the valuable metals by wet treatment have been proposed. However, even in such methods using wet treatment, dry treatment is partly used, and realization of low cost is difficult because of complexity of a treatment process. Therefore, the valuable metals cannot be efficiently recovered.
Especially, regarding lithium of a valuable metal, there is a problem that impurities, such as phosphorus and fluorine, are mixed in, and thus high-quality lithium cannot be efficiently recovered in the form of a simple substance. To be specific, a lithium ion battery contains, as an electrolyte, lithium hexafluorophosphate (LiPF6) and the like that constitute lithium that is a valuable metal. This lithium hexafluorophosphate has a hydrolysis reaction through wet treatment, and forms a precipitate in the forms of lithium phosphate (Li3PO4) and lithium fluoride (LiF), and lithium cannot be efficiently recovered in the form of a simple substance.
While hexafluorophosphate ions in an electrolyte solution do not form slightly soluble salts with metal ions other than potassium and aluminum, the hexafluorophosphate ions form the slightly soluble salts with the majority of metal ions when hydrolyzed and changed into phosphate ions and fluoride ions. When separation and refinement treatment is performed in the coexistence of these hydrolysates, ions of these hydrolysates are precipitated on products, resulting in quality failure.
As a method for removing hexafluorophosphate ions, for example, Patent Literature 2 describes a method for adding potassium fluoride and ammonium fluoride, forming slightly soluble hexafluorophosphate and lithium fluoride, and separating the hexafluorophosphate and lithium fluoride as a precipitate. However, the technology described in Patent Literature 2 has problems that phosphorus, fluorine, and lithium are recovered as a coprecipitation mixture, already hydrolyzed phosphate ions cannot be separated, and excessively added fluoride is remained in mother liquor.
Further, Patent Literature 3 discloses a method for absorbing hexafluorophosphate ions with a basic ion exchange resin, preferably a weak basic ion exchange resin. However, because behavior of the already hydrolyzed phosphate ions and fluoride ions is different, there is a limit to remove the phosphate ions and the fluoride ions at the same time, and even the technology of Patent Literature 3 cannot sufficiently remove the phosphate and the fluoride.
Meanwhile, Patent Literature 4 discloses a method for remaining hexafluorophosphate ions in an extraction residual liquid by using a positive ion exchange-type acidic extractant, and selectively extracting and separating only lithium ions. However, even the technology described in Patent Literature 4 has problems that, in a process of adjusting a solution to have necessary pH for extraction, the hexafluorophosphate ions are hydrolyzed and a precipitate of lithium phosphate and lithium fluoride is generated, becomes crud, and is physically mixed in an extraction solvent, and lithium is contaminated.